1. Field of the Invention
The invention relates generally to the field of drilling wellbores through subsurface rock formations. More specifically, the invention relates to methods for controlling wellbore pressure during assembly or disassembly of lengths of drill pipe.
2. Background Art
Drilling wellbores through subsurface rock formations includes rotating a drill bit disposed at the end of a drill pipe disposed in the wellbore. Various devices are used to rotate the pipe and/or the bit while pumping drilling fluid through the pipe. The drilling fluid performs several functions, namely to cool and lubricate the bit, to lift drill cuttings out of the wellbore, and to provide hydraulic pressure to maintain wellbore mechanical stability and to restrain fluid under pressure in various permeable subsurface formations from entering the wellbore.
It is known in the art to use drilling fluid having lower specific gravity than that which would exert sufficient hydraulic pressure to retain fluids in such formations. One such technique is described in U.S. Pat. No. 6,904,981 issued to van Riet and commonly owned with the present invention. Generally, the system described in the '981 patent uses a rotating diverter or rotating control head to close the annular space between the drill string and the wellbore wall. Flow out of the wellbore is automatically controlled so that the fluid pressure at the bottom of the wellbore is maintained at a selected amount.
The drill pipe is assembled from a number of individual segments (“joints”) of pipe threadedly coupled end to end. In order to lengthen the wellbore, it is necessary from time to time to add joints to the drill pipe. To remove the drill pipe from the wellbore, for example to replace the drill bit, it is necessary to threadedly disconnect sections (“stands”) of the drill pipe from the part of the drill pipe remaining in the wellbore. When using the system described in the van Riet '981 patent, for example, it is desirable to include a one way (“check”) valve in the drill pipe so that when the upper part of the drill pipe is opened, i.e., disconnected from a kelly or top drive, drilling fluid is prevented from flowing back up the drill string. Annulus pressure can be maintained using a back pressure pump, or by diverting some of the flow from the drilling unit fluid pumps into the annular space.
U.S. Pat. No. 6,823,950 issued to von Eberstein, Jr. et al describes a technique for maintaining wellbore pressure during connections for marine drilling systems in which a wellhead is located at the sea floor and a riser fluidly connects the wellbore to a drilling unit on the water surface. The method disclosed in the '950 patent requires filling an auxiliary fluid line associated with the riser system with higher density fluid and/or applying pressure to such line to maintain a selected fluid pressure in the wellbore.
A particular disadvantage of using the method described in the '950 patent is that switching from drilling to maintaining wellbore pressure during connections is that it requires the drilling unit operator exercise a high degree of care during the transition from drilling using the drilling unit pumps to the conditions necessary required to make a connection. There may be risk, for example of u-tubing because of the higher density fluid being inserted into the auxiliary line. This may create risk of exceeding formation fracture pressure at some point in the wellbore.
What is needed is a technique for maintaining wellbore pressure during the transition from drilling to making connections and during connections that does not require the use of higher density fluid in the auxiliary lines.